Closed pond system for wet process phosphate plants

ABSTRACT

CLOSED POND SYSTEM OR PROCESS FOR ELIMINATING THE CONVENTIONAL POND SYSTEMS OF WET PROCESS PHOSPHORIC ACID COMPLEXES, AND TO REMOVE FLUORINE FROM PROCESS GAS STREAMS, WHEREIN LIQUID EFFLUENTS FROM WET PROCESS PHOSPHORIC ACID COMPLEXES, INCLUDING SCRUBBER LIQUOR FROM GAS SCRUBBING OPERATIONS OF THE COMPLEX, ARE COOLED AND CLARIFIED, AND THE LIQUID FROM THE CLARIFIER, NEUTRALIZED WITH AMMONIA, IS RECYCLED TO THE PHOSPHATE COMPLEX, THE SLUDGE OR SLURRY BEING FILTERED TO REMOVE SOLIDS, THE SOLIDS BEING CALCINED TO DRIVE OFF FLUORINE-CONTAINING GASES. THE CALCINED SOLIDS CONSIST PRINCIPALLY OF SODIUM FLUORIDE (NAF) WHICH RECYCLED TO THE CLARIFIER OVERFLOW TO BE MIXED WITH THE LIQUID RETURNED TO THE PHOSPHATE COMPLEX. IN THE PHOSPHATE THE AMMONIATED LIQUID FROM THE CLARIFIER IS USED TO SCRUB PLANT GASES, AND IS MIXED WITH OTHER SCRUBBER LIQUORS FROM THE PLANT, AND RECYCLED AGAIN TO THE COOLING TOWER AND CLARIFIER. THE HF IN THE EFFLUENT PLANT LIQUIDS IS CONVERTED TO SIF4 BY MAINTAINING AN EXCESS OF SIO2, AND THE NAF REACTS WITH THE SIF4 TO FORM NA2SIF6. THE NA2SIF6 IS CALCINED TO PRODUCE NAF AND GASEOUS SIF4, THE LATTER BEING SCRUBBED WITH WATER TO FORM H2SIF6 OF COMMERCIAL QUALITY.

Feb. 19, 1974 R, G. HARTIG 3,793,436 CLOSED POND SYSTEM FOR WET PROCESS PHOSPHATE PLANTS Filed Nov. 24.-, 1971 2 Sheets-Sheet 2 .gf/gm l U.S. Cl. 423-341 3 Claims ABSTRACT OF THE DISCLOSURE Closed pond system or process for eliminating the conventional pond systems of wet process phosphoric acid complexes, and to remove uorine from process gas streams, wherein liquid eiuents from wet process phosphoric acid complexes, including scrubber liquor from gas scrubbing operations of the complex, are cooled and clarified, and the liquid from the clarifier, neutralized with ammonia, is recycled to the phosphate complex, the sludge or slurry being filtered to remove solids, the solids being calcined to drive off iiuorine-containing gases. The calcined solids consist principally of sodium uoride (NaF) which is recycled to the clarifier overow to be mixed with the liquid returned to the phosphate complex. In the phosphate the ammoniated liquid from the clarifier is used to scrub plant gases, and is mixed with other scrubber liquors from the plant, and recycled again to the cooling tower and clarifier. The HF in the effluent plant liquids is converted to Si=F4 by maintaining an excess of SiO2, and the NaF reacts with the SiF., to form Na2SiF6. The Na2SiF6 is calcined to produce NaF and gaseous SiF4, the latter being scrubbed with water to form H2SiF6 of commercial quality. i

This application is a continuation-in-part of application Ser. No. 60,226, tiled Aug. 3, 1970, of the same title, now U.S. Pat. 3,720,757.

BACKGROUND OF THE INVENTION This process is designed to eliminate all contamination by gaseous iiuorine liberated in phosphate rock processing from the water used to slurry gypsum, in wet process phosphoric acid complexes, and to remove fluorine from process gas streams. P205 is recovered for recycling to the plant as NH4H2PO4. In addition, various other benefits are obtained including uorine recovery as a saleable product, elimination of liming costs, higher P205 recovery, and the like.

In current practice, byproduct gypsum from wet process phosphoric acid complexes is slurried with water and pumped to a pond of vast acreage in which the gypsum settles out and the supernatant water is cooled in ponds and recycled to process for use as scrubbing water to remove fluorine from process gas streams, as condensing streams in barometric condensers, and to slurry gypsum. The iiuorine content of this water builds up to appreciable concentrations presenting undesirable problems, such as surface water and ground water contamination by fluorine. Addition of lime to these waters is frequently resorted to in order to control pollution of surface waters into which the plant waters may become intermingled.

.United States Patent O rice By the novel process herein presented, it is possible t0 eliminate this type of iluorine contamination by completely eliminating the use of gypsum slurry water in scrubbing and processing. All fiuorine contaminated scrubbing liquor is handled and stored in impervious, lined equipment separate from the gypsum slurry system.

The liquid solution circulated through this process and the complex has a pH of 1.5-4.7, and therefore has no fluorine vapor pressure because the fluorine therein is present as the two stable salts, |Na2SiF6 and NaF. This condition precludes the presence of any uorine in the air exits of cooling towers, and in other equipment. The total gaseous fluorine emission from the entire closed pond system will be less than 15 pounds per day, for a phosphate complex of the size wherein the volume of liquid eliiuent is in the 16,000 gallons per minute range.

Other objects and advantages of the process herein disclosed will appear from the following detail description of a preferred embodiment, reference being made to the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS FIGS. lA and 1B are flow diagrams which together show a preferred embodiment of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT lReferring first to FIG. l, the effluent leaving the phosphate complex enters the process at flowline 10, which leads into catch basin 11. The liquid contains approximately 0.56 percent fluorine, and is at a temperature of between F. and 120 F. The uorine is present as NazSiF which has been formed by the reaction between NaF and SiF4. A slight excess of reactive Si02 is maintained in the recirculating liquid stream to convert any HF to The liquid also contains a small amount of NaF in solution and contains ammonium phosphate salt in the monoammonium form, NH4H2PO4. The NH4H2PO4 solution is recirculated through the phosphate complex scrubber system, and the concentration is maintained by continuous removal of NHHPO solution from the system at the solids removal stage.

The liquid is pumped from catch basin 11 by cooling tower lift pump 12, the flow beingthrough fiowline 13 containing valve 14 and, upon leaving the pump, through valve 16 and ilowline 17 which delivers the liquid to the top of cooling tower 18.

The liquid passes downwardly through cooling tower 18, while air is drawn in through plural side inlets 19 to pass upwardly through mist eliminator 20, the air being drawn upwardly by a blower 21, driven by a motor, not shown. The cooling tower may be packed or equipped with baffles, or the like, in any suitable manner. Any suitable cooling tower may be substituted.

The cooled liquor which leaves the cooling tower by gravity through flowline 23 is at a temperature of 90 F., maximum. The liquid is delivered through owline 2.3` into the inlet 24 of clarifier 25. The clarifier may be of any suitable form. Claritier 25 as shown includes paddles or drags 26 which rotate in order to collect the thickened slurry at the lower center of the clarifier. The clarifier has a downwardly converging bottom or oor, and has therearound an overflow trough 27 in fiow communication with an overfiow sump 28. Liquid overflow from the clarifier exits through flowline 29 containing valve 30, pumped by pump 31 through iiowline 32 having valve 33 which delivers the overflow liquid from the clarifier back to the phosphate complex, wherein the liquor is used for scrubbing and/or other plant purposes, the liquid being circulated through the complex to be returned to the herein described process through fiowline 10.

Fresh water is added as necessary to the overflow sump 28 through valved -flowline 37.

The overflow liquor from the clarifier contains approximately 0.5% fluorine, present as Na2SiF6, and a small excess of NaF in solution. This concentration of fluorine is approximately 0.06% less than the concentration of fluorine in the liquid entering the process through owline 10. The excess iiuorine in iiowline is derived from the phosphate complex scrubbing and washing operations.

The liquor coming into overfiow trough 27 and sump 28 contains phosphate compounds in solution, partly in the form, H3PO4, and partly in the previously neutralized form, NH4H2PO4. Ammonia is added in the overflow sump 28 through fiowline 38 to convert the H3PO4 to NH4H2PO4, so that the phosphate content will be returned in soluble form to the phosphate complex. The chemical reaction of H3110., with ammonia is:

A water slurry of SiOz may be introduced to the clarifier overflow sump 28 through a flowline 53 in order to maintain the SiO?, concentration in the liquid recirculated therefrom to the phosphate complex.

The slurry leaving clarifier 25 at the center bottom thereof exits through iiowline 39 having 'valve 40, being moved by pump 41 delivering the slurry through flowline 42 to a filter 54. The filter 54 is indicated in the drawing as being a rotary drum filter, but other suitable filtration apparatus may be suitably used. The filtrate containing NH4H2PO4 from filter 54 is discharged through fiowline 55 by gravity to sump 56, to be delivered to the phosphate complex by pump 57.

The solid materials collected at filter 54 are delivered by conveyor 58 to the feed 59 of a calciner 60. The calciner 60 is indicated in the drawing as being of the rotary drum, countercurrent flow type, but other suitable apparatus for this purpose may be substituted. Fuel and air are burned at burner 62 of calciner 60, the flue gas passing toward the left through the calciner to be discharged through rliowine 63.

In calciner 60, Na2SiF6 is decomposed at a temperature of 1350 F. to produce solid NaF and gaseous SiF4. 'I'he gaseous SiF., is discharged from calciner 60 through owline 63 with the flue gases from the burner. A portion of the liquid in owline 32 is diverted through fiowline 65 to be introduced into NaF slurry tan-k 66, the amount introduced being controlled by valve 67. Slurry from tank 66 is circulated through flowline 69 including Valve 70` by pump 7'1 which delivers the slurry through liowline 72 including valve 73 through the discharge 76 of calciner 60. Calcined NaF is discharged from the calciner to be mixed with the circulating slurry at 76. The slurry-solids mixture is passed through Iflowline 77 to the slurry tank 66.

A portion of the slurry in flowline 72 is passed through owline 79 to be delivered into the clarifier overflow sump 28. A portion of the slurry passing through 'fiowline 79 is circulated through fiowline 80 having lvalve 81 through the bottom of calciner feed 59, to be delivered through fiowline 82 for mixture with the slurry mixture in iiowline 77. In this way, material at the calciner feed dust hood may be mixed with the slurry in slurry tank 66.

The flow through flowline 79 delivers make-up NaF to the closed pond circulating liquid returned to the phosphate complex through flowline 32.

The gases discharged from calciner 60 pass through owline 63, through scrubber 89. After passing through scrubber 89, the gases are passed through diowline r91 to a second scrubber 92, and thence through flowline 93- to a third tail tower scrubber 95. From scrubber 95 the gases pass through liowline 96 to blower 97, and thence through lfiowline 98 having valve 99 to stack 100'. The gases leaving calciner 60 through flowline 63 contain Sil-74 derived from decomposition by heat of NagSiF according to the reaction,

Fresh water is introduced to scrubber '92 to wash the gases, through sfiowline 10'3. The scrubber liquor from tower 92 exits through valve 104 and lflowline 105 through pump 106 which delivers part of the liquid in recirculation through line 107 having valve 108, and which delivers a part of the liquid through flowline 109, ow through which is controlled by lvalve 110, into scrubber 89 wherein the scrubber liquor is contacted with the gases passing through the scrubber. The scrubber liquor is discharged from scrubber 89 through valve 114 and flowline 115, and is recirculated to the top of scrubber 89 through iiowline 116, being propelled therethrough by pump 117. Flow through fiowline 116 is controlled by valve i118. Scrubber slurry is withdrawn from this system through fiowline 120 having valve 121.

Flowline branching from owline 65 carries overfiow liquid from clarifier 25 to the top of scrubber 95, owline 145 having a control valve 146. Recirculating liquor in tower 95 drains through valve 147 and flowline 148 to be pumped by pump 149 through recirculation fiowline 150, flowline 150 having valve 151. Scrubber liquor is withdrawn from flowline 150 through valve 153 and flowline 154 to be delivered into the feed inlet of clarifier 25. Liquid condensing in blower 97 is drawn through owline 156 back to scrubber 95. The scrubber has a mist eliminator 157 through which the exit gas passes into iiowline 96.

In the liquid circulation stream through cooling tower 18, clarifier 25, and the phosphate complex, any phosphoric acidpicked up by the liquor reacts with ammonia to form NH4H2PO4. The chemical reaction is:

SiF4 is recovered in scrubber 92, and tail scrubber 95 picks up any residue so that the gases passing out of stack 100 are essentially fiuorine free. The scrubber liquor passing through owline 120 is delivered through a filter 190, which is indicated to be of the rotary drum type, but for which any suitable type of filtration apparatus may be substituted. The filtrate passes through barometric leg flowline 191 to seal tank 192. This filtrate is a Water solution of`H2SiF6, which is delivered through pump 194 and valve 195 through iiowline 196 to storage. This solution is a valuable commercial product.

The filter cake from filter 190, consisting of Si02 in pure form is delivered by conveyor 197 to slurry tank 198 to which water is added through fiowline 199 controlled by valve 200. The SiO2 slurry is delivered through valve 203 by pump 204 through valve 205 and fiowline 206 to the plant, for addition of SiO2 to the plant process where needed or for other use. A part of the Si02 slurry is delivered through valve 208 and flowline 53, previously described, to the clarifier overow sump 28.

Approximately 16,000 gallons per minute of plant eiuent is delivered in a typical phosphate complex through fiowlines 10 and 17 to cooling tower 18, the liquid being of a specific gravity of approximately 1.1-1.2, at a temperature of approximately 109 F. The cooled efuent in flowline 23 is at the same fiow rate less evaporation loss and has the same gravity, and its temperature is approximately 90 F., maximum.

In owline 32, approximately 16,000 gallons per minute of return liquor is returned to the phosphate complex,

at a specic gravity of about 1.1-1.2 and a temperature of about 90 F., or less.

Operating data are shown for a typical plant in the following Table I, providing an example of the process:

6 (b) separating said NazSiF from said liquors, (c) returning the liquors from which said NazSiFs has been separated to said complex for reuse in at least one of said scrubbing and washing operations,

TABLE I TYPICAL OPERATING CONDITIONS Flow rate Tem- Flow rate through pera- NazSiFe H2O, through SiF4, NaF, SiF4, Flowowline, ture, NH4H1PO4 (as F), NaF, S101, NazSllie. pertlowline, per- HzSiF, 1bs./ 1bs./ HgSiFa, line gah/min. F. (as'PzOs) percent percent percent lbs./ru1n. cent s.c.f./min. cent lbs/min. min. min. percent 10 13,000 106 10.0 0.5 SLXS SLXS 17--.--. 12,850 106 10.0 0.5 SLXS SLXS 23---.-- 12,850 90 10.0 0.5 SLXS SLXS 79-.--.- 82 20.0 58... 63,-- 600 77 82 120 47 160 92 160 1m 57 23 10.00 196 46 150 NaF is recovered for reuse. Phosphate values are recycled to the plant as NH4H2PO4. 'Ihe gases from the process are essentially free of uorine, and no water solutions are disposed of to result in contaminations of surface and ground Waters.

While a preferred embodiment of the invention has been shown in the drawings and described, many modiications may be made by a person skilled in the art without departing from the spirit of the invention, and it is intended to protect by Letters Patent all forms of the invention falling within the scope of the following claims.

I claim:

1. Process for continuously removing SiF., from a stream of liquors containing 'SiF4 and H3PO4 which is eflluent from at least one of scrubbing and washing operation of a Wet process phosphoric acid complex and which can be returned immediately for reuse in at least one of said scrubbing and washing operations of said complex following said removal of SiF4 thereby avoiding necessity of use of the conventional pond systems of such complexes, comprising (a) continuously adding NaF and NH3 of said stream of said liquors euent from said complex to convert the SiF, to NazSiFs according to the chemical reaction,

SiF42NaFP') NagSiFe and to convert the H3PO4 to NH4H2PO4 according to the chemical reaction,

(d) calcining the separated Na2SiF6 to form NaF and SiF4 according to the chemical reaction,

(heat) Nassim zNaF-i-SiFi (e) adding the NaF recovered in step (4) to additional liquors according to step (a),

(f) and scrubbing the SiF4 recovered in step (d) with water to recover a solution of HZSFS.

2. Process according to claim 1, wherein said liquors also contain fluorine values in the form HF and comprising adding SiO2 to the liquors to convert the HF to H2SiF6 according to the chemical reaction,

3. Process according to claim 1 wherein said liquors are cooled prior to said separation of NaZSiFB therefrom.

References Cited UNITED STATES PATENTS 2,447,359 8/ 1948 Oakley, Ir. 423-341 X 2,556,064 6/ 1951 Caldwell et al 423-341 2,785,953 3/ 1959 Fitch 423-341 X 3,201,195 8/1965 Hubert et al 423-184 3,256,061 6/1966 Tufts et al. 423-341 X 3,415,039 12/1968 Rushton et al. 423-341 X 3,420,621 1/1969 Barker 423-341 X 3,507,799 4/1970 Hart 423-341 X 3,533,737 10/1970 Farr et al. 423-305 3,551,098 12/ 1970 Flemmert 423-499 3,615,195 10/1971 Bierman, Jr. et al. 423-341 X EARL C. THOMAS, Primary Examiner G. A. HELLER, Assistant Examiner U.S. Cl. X.R. 423-3 11 

